In a differential pair, the difference of output currents depends on the difference of input voltages, and if a null potential difference is applied between the inputs, for example by short circuiting the inputs, the difference of the output currents should be null.
The defects of matching the components that constitute the pair mean that the pair has an offset input voltage, more commonly called the offset voltage, that is not null; this offset voltage represents the intrinsic imbalance of the pair and is reflected by a difference of non-null output currents in the presence of a null input differential voltage. In practice, the offset voltage is the compensation voltage that needs to be applied between the inputs so that the difference of output currents is null.
A differential pair conventionally comprises two branches supplied by one and the same reference current source Io, each branch comprising a transistor and a charge. The input voltages are applied to the bases of the transistors, the emitters are connected to the common current source, and the collectors are connected to the charges. The current of the source is shared between the two branches depending on the difference of input voltages applied to the bases.
The differential pairs may be used individually or grouped into associations of pairs when the installations are more complex. For example, two pairs may share one and the same charge, the collector of a transistor of one pair being connected to the collector of a transistor of another pair. The pairs may be mounted in cascade, a current output of one pair being connected to a current output of another pair which itself has another current output connected to a third pair, and so on. Depending on the use of the differential pairs, it is possible to find different associations of pairs. The invention applies in general to all these uses.
Such differential pairs are used notably in analog-digital converters. They are used for example to constitute comparators, each comparator comprising a differential pair receiving as an input, on the one hand, a voltage to be converted, on the other hand a reference voltage; they are also used, still for converters, in folding circuits; a folding circuit comprises several folding cells each constituted of at least one differential pair, the current outputs of the cells being connected in cascade to one another in order to establish a voltage or an analog output current that varies in a bell or in a sinusoid depending on the input voltage to be converted; the different cells each receive the input voltage and a respective reference voltage.
In precision analog circuits using several differential pairs, it is realized that the lack of precision of operation (notably lack of precision of conversion in the converters) may result from the presence of non-null offset voltages; from a physical point of view, these offset voltages result above all from the fact that the emitter-base voltages of the various transistors of the pairs are not exactly identical even when identical currents pass through them.
Specifically, the technologies are not perfect and two transistors manufactured simultaneously, having at least theoretically the same emitter surfaces, and even placed side by side in an integrated circuit therefore having every chance of being identical, do not have strictly identical features. This results from an inevitable dispersion of manufacturing. In addition, not only do the two transistors of a pair generate an offset voltage in this pair, but, due to this very dispersion, the various differential pairs of an integrated circuit inevitably have offset voltages that are different from one another.
It has already been proposed to reduce this disadvantage by using differential pairs having larger transistors. They have less dispersion because their size is better controlled. But then, the capacitors are bigger and the circuits are therefore slower, which is not desirable in applications such as fast analog-digital converters. For the latter, it would be better to have smaller transistors in the differential pairs. In addition, the circuits are more bulky if the transistors are bigger, and they consume more current.
Individual calibration solutions have also been proposed a posteriori: a manufactured converter is tested and the conversion errors are stored in an EPROM memory of the integrated circuit to be used for the compensation of the errors during use. Laser adjustment solutions also exist, notably for individually adjusting the resistances present in the differential branches. This technique requires an individual test and an individual correction of each integrated circuit depending on the conversion errors noted. The method is therefore extremely costly industrially, each circuit having to be tested and corrected individually.